5901 23RD DR W Geotech Report 2025-07-29

I October 22,2007 Project No.T-5922-1 Our study indicates that the near-surface native soils contain a sufficient percentage of fines (silt and clay size particles) that will make them difficult to compact as structural fill if they arc too wet or too dry. Accordingly, the ability to use these soils from site excavations as structural fill will depend on their organic and moisture contents and the prevailing weather conditions when site grading activities take place. Native soils that are too wet to properly compact could be dried by aeration during dry weather conditions or mixed with an additive such as cement, cement kiln dust (CKD), or lime to stabilize the soil and facilitate compaction. If soil amendment products are used, additional Temporary Erosion and Sedimentation Control (TESC)Best Management Practices (BMPs) will need to be implemented to mitigate potential impacts to stormwater runoff associated with possible elevated pH levels. If grading activities are planned during the wet winter months, or if they are initiated during the summer and extend into fall and winter, the contractor should be prepared to import wet weather structural fill. For this purpose, we recommend importing a granular soil that meets the following grading requirements: U.S.Sieve Size Percent Passing I 6 inches 100 No.4 75 maximum No.200 5 maximum* *Based on the 3/4-inch fraction. Prior to use, Terra Associates, Inc. should examine and test all materials to be imported to the site for use as structural fill. Structural fill should be placed in uniform loose layers not exceeding 12 inches and compacted to a minimum of 95 percent of the soil's maximum dry density, as determined by American Society for Testing and Materials (ASTM) Test Designation D-698 (Standard Proctor). The moisture content of the soil at the time of compaction should be within two percent of its optimum,as determined by this same ASTM standard, In nonstructural areas, the degree of compaction can be reduced to 90 percent. 5.3 Slopes and Embankments All permanent cut and fill slopes should be graded with a finished inclination of no greater than 2:1. A fill placed for embankment construction should meet the structural fill requirements in Section 5.2 of this report. Upon completion of grading, the slope face should be appropriately vegetated or provided with other physical means to guard against erosion. Final grading should direct surface runoff away from the slope crest. Surface water from developed areas must not be allowed to flow in an uncontrolled and concentrated manner over the crest of the steep slope. Surface water should be directed away from the slope crest to a point of collection and controlled discharge. If site grades do not allow for directing surface water away from the slope, then the water should be collected and tightlined to an approved point of controlled discharge. 5.4 Excavations All excavations at the site associated with confined spaces, such as lower building level retaining walls, must be completed in accordance with local, state, and federal requirements. Based on the Washington State Safety and Health Administration (WSHA) regulations, the upper medium dense soils would be classified as Type C soils. Very stiff to hard silt and dense to very dense till-like soils would be classified as Type A soil. I Page No. 8 I